I love Joss Whedon's work and the cast is wonderful, but...

if I had a say in casting, I would make some changes to the cast. These would be my choices.
(Note:All of the actors mentioned have worked with Joss Whedon.)

Nicholas Brendon (Xander on Buffy the Vampire Slayer) as Benedick - Benedick has a lot of witty lines in the play, and since Nick was the reigning class clown and jokester in Sunnydale, I think that he'd have made an excellent Benedick. Also, Xander was one of my favorite characters on Buffy, and I'd give anything to see Nicholas and Joss work together again. That, and the fact that Nicky hasn't had a steady acting career since Buffy ended.

Emma Caulfield (Anya on Buffy) as Beatrice - The relationship between Beatrice and Benedick is the focal point of Much Ado About Nothing, and if there was one actress who could match Nicholas, it would most definitely be Emma. What their characters on Buffy had is very similar to what Beatrice and Benedick had. Emma, like Nicholas, also had some of the best lines on Buffy, and I think that she'd bring to Beatrice what she brought to the role of Anya.

(Side note: I love Amy and Alexis and I think they would do justice to their roles, but somehow, I just can't picture them having the same banter that Nick and Emma can do. Wesley and Fred weren't really that kind of couple, you know.)

J. August Richards (Gunn on Angel) as Don Pedro - This is a case of pure tokenism on my part; Don Pedro was played by an African-American (Denzel Washington) in the 1993 film; somehow, I don't think I could see the character as anyone else. And since J. August Richards is African-American...

Morena Baccarin (Inara on Firefly/Serenity) as Ursula - Because I wanted another Firefly alum in this film. (except Adam Baldwin; I'm not exactly a fan)

Ron Glass (Book on Firefly/Serenity) as Friar Francis - He played a preacher on Firefly; it would be nice to see him play that same kind of role again (albeit in a different Whedon work)

Of course, I know this is Whedon's project, not mine. He was the one who chose the actors, and I know that they'll be perfect in this film. I'm not trying to incite criticism; I'm merely giving my own input.